Heterodyne types of radio receivers use multiple oscillator frequencies to convert incoming RF signals from the RF carrier frequency to a lower frequency at which demodulation is performed. In this process, the signal may be down-converted from RF to one or more intermediate frequencies (IFs) before being demodulated. The demodulation can be performed at an IF frequency (e.g., 455 kHz, 10.7 MHz, etc.) or at a baseband frequency using the I-Q complex representation of the signal. The overall receiver stability is a function of the sum of the stabilities of all oscillators used in the frequency conversions.
Ideally, the conversion frequencies should be synthesized from a single, high-accuracy oscillator. However, this is not always the most cost-effective approach where low receiver cost is a requirement. Even where a high-accuracy oscillator is used for down-converting from RF to IF, system requirements or design considerations may dictate the use of certain frequencies for subsequent down-conversion stages that cannot be derived from the high-accuracy oscillator. For example, the frequencies of intermodulation products resulting from down conversion may rule out use of certain frequencies or combinations of frequencies in the down conversion scheme. Another factor in selecting conversion frequencies is the availability of off-the-shelf components at certain frequencies. Where the down-conversion frequencies cannot all be derived from a single oscillator, cost considerations may prohibit the use of additional high accuracy oscillators at these other frequencies. Consequently, many lower-cost designs use one or more inexpensive crystal oscillators for the lower oscillator frequencies, where the frequency error introduced by the oscillator will be less. Because manufacturers specify the stability of crystal oscillators in parts-per-million (or ppm), lower crystal frequencies introduce less total frequency error.
One drawback of current designs that employ lower-cost oscillators is that these oscillators degrade receiver performance. A typical tradeoff is to increase receiver bandwidth to accommodate frequency errors introduced by the lower accuracy oscillators. A wider bandwidth helps to ensure that down-converted signals remain within the receiver bandwidth despite being offset from the center of the band as a consequence of oscillator-induced frequency errors. However, increased bandwidth decreases receiver performance with weak signals and rejection of adjacent channel interference. In some cases, if the frequency error is too large, the receiver phase demodulators may not even work. Accordingly, there remains a need for receiver designs and techniques that minimizing frequency errors introduced by low accuracy oscillators in RF receivers such as radio receivers.